The present invention relates to an electrographic development process, and more particularly to an electrophotographic development process employing a developer comprising electrically conductive particles and toner particles with high resistivity (hereinafter referred to as high resistivity toner particles).
Conventionally, in the field of electrostatic recording, a development process is known wherein a developer is electrically charged to a certain polarity and is brought into contact, or proximity, with an image pattern of electric charges with a polarity opposite to the polarity of the developer, whereby the electrically charged image pattern is developed to a visible image by the developer being electrically attracted thereto.
It is required that the developer for use in this process meet two contradictory conditions, i.e., it must both allow easy charge injection and provide high charge retention. In order to meet those requirements, a development process as partly shown schematically in FIG. 1 has been proposed, in which a developer 2 is magnetically or electrically attracted to a donor roller 1 and is held thereon. Electric charges with a predetermined polarity are injected into the developer 2 held on the donor roller 1, so that the developer 2 is electrically charged to that predetermined polarity. The developer 2 is then brought into contact with a latent electrostatic image with a polarity opposite to that of the developer 2, formed on a latent electrostatic image bearing member (not shown), whereby the latent electrostatic image is developed when the developer 2 is electrically attracted thereto.
Two variations of this proposed development process are known. In the first type, there are disposed separately a blade member for regulating the thickness of the developer on the donor roller 1, and an electrode for injecting electric charges into the developer, while in the second type, a doctor blade, which serves as the blade member and as the electrode, is used. In the development process shown in FIG. 1, a doctor blade 3 serves for regulating the thickness of the developer 2 on the donor roller 1 to a predetermined thickness and, at the same time, for injecting electric charges into the developer 2 from a power source 4.
In this process, it is known that the speed of charge injection from the doctor blade 3 to the developer 2, which is defined as the number of electrons which flow from the doctor blade to the developer per unit time, and the quantity of electric charges held in the developer 2, increase as the pressure applied by the doctor blade 3 to the developer 2 and the quantity of the developer 2 on the donor roller 1 increase.
In the present invention, it is necessary that the developer allow electrons to flow easily thereinto, yet still trap electrons effectively.
In order to increase the quantity of charges trapped by the developer from the viewpoint of the above-mentioned necessity, the developer must have high electric resistivity, and a high voltage must be applied to the developer through the doctor blade. However, if the voltage applied is high, there is a risk that the developer layer formed on the donor roller 1 will be destroyed by excess charging, or spark discharging may take place in the neighboring parts, resulting, among other problems, in the so-called blank "halos" in the solid portion of the developed images.
Under such circumstances, in order to obtain electric charges sufficient for development in this process, conventionally the electric resistivity of the developer mass as a whole is decreased by applying pressure to the developer through a blade and thickly packing the developer layer. The aim of this decreasing of the electric resistivity of the developer is to achieve easy and stable charge injection into the developer. However, it is extremely difficult in practice to attain stable charge injection by such mechanical adjustment of the blade. As a matter of fact, that method is far from being of practical use.